Characterization of a vertebrate neuromuscular junction that demonstrates selective resistance to botulinum toxin

Botulinum toxin blocks transmitter release by proceeding through a series o f four steps: binding to cell surface receptors, penetration of the cell me mbrane by receptor-mediated endocytosis, penetration of the endosome membra ne by pH-induced translocation, and intracellular proteolysis of substrates that govern exocytosis. Each of these steps is essential for toxin action on intact cells. Therefore, alterations in cell structure or cell function that impede any of these steps should confer resistance to toxin. In the pr esent study, screening for susceptibility to four serotypes of botulinum to xin revealed that the cutaneous-pectoris nerve-muscle preparation of Rana p ipiens is resistant to type B botulinum toxin. Resistance was demonstrated both by electrophysiologic techniques and by dye-staining techniques. In ad dition, resistance to serotype B was demonstrated at toxin concentrations t hat were 2 orders of magnitude higher than those associated with blockade p roduced by other serotypes, In experiments on broken cell preparations, typ e B toxin cleaved synaptobrevin from frog brain synaptosomes. However, the toxin did not bind to frog nerve membranes. These findings suggest that res istance is due to an absence of cell surface receptors for botulinum toxin type B. The fact that cutaneous-pectoris preparations were sensitive to oth er botulinum toxin serotypes (A, C, and D), as well as other neuromuscular blocking agents (alpha-latrotoxin, beta-bungarotoxin), indicates that botul inum toxin type B receptors are distinct.